Machine element of surface-hardened steel having an improved resistance against wear, heat, and mechanical stress

ABSTRACT

A machine element of surface-hardened steel with improved resistance against wear, heat and mechanical stress which is obtained by using a high-alloy tool steel and by heat treating the part through heating of its surface in several passes of induction heating or flame heating above the transformation point and through subsequent quenching.

United States Patent 11 1 Hehl 1 1 MACHINE ELEMENT OF SURFACE-HARDENED STEEL HAVING AN IMPROVED RESISTANCE AGAINST WEAR. HEAT, AND MECHANICAL STRESS [76] Inventor: Karl Hehl, Sicdlung 183, 7291 Lossburg, Wurttemberg. Germany [221 Filed: July 5, 1973 [21] Appl. No: 376,292

[30] Foreign Application Priority Data July 5, 1972 Germany 2232932 [52] US Cl. 1. 148/124; 148/147; 148/150;

[51] Int. Cl. ..C2Id1/42;C21d 9/00 [58] Field of Search 148/315, 39. 146, 150.

[56] References Cited UNITED STATES PATENTS 1,963,403 6/1934 Daniels H 148/39 1451 May 20, 1975 2,669,647 2/1954 Segsworth i i i i. 219/1071 3.761.370 9/1973 Keller 148/166 FOREIGN PATENTS OR APPLICATIONS 572.376 10/1945 United Kingdom 148/151 573.751 4/1959 Canada 148/151 OTHER PUBLICATIONS Tool Steels Gill. 1944, ASM pgs. 189 & 1901 The Selection and Hardening of Tool Steels. McGraw Hill C0., 1950. pgs. 109-121 Primary Examiner-C4 Lovell Armrney, Agent, or Firm-Joseph A. Geiger [57] ABSTRACT 8 Claims, No Drawings PATENTED HAY 20 75 T m ma Fm Du P INVENTOR JOSEPH S. STACK AORNEYS MACHINE ELEMENT OF SURFACE-HARDENED STEEL HAVING AN IMPROVED RESISTANCE AGAINST WEAR, HEAT, AND MECHANICAL STRESS BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the production of steel parts which serve as highly stressed machine elements or the like, and in particular to parts made of alloy steel which have a hard surface giving them a high wear resistance and a tough core giving them resistance against mechanical stress, such as tension, compression, torsion, and impact forces. Machine elements produced in accordance with the present invention are particularly suitable for use under conditions in which they are exposed to hot, abrasive substances, such as mineral masses which are processed in pressure molding machines for the production of sintered parts, or in the injection molding field, when the plastic masses contain considerable amounts of mineral filler materials.

2. Prior Art In the above-mentioned field of application with which the present invention is concerned, the machine elements in question are frequently subjected not only to heat and wear, but also to very high mechanical stresses resulting from impact forces, bending, torsion and related load conditions. It is common knowledge that only steel parts with a very hard surface can effectively withstand the wear and abrasion tendency of mineral masses, and it is also known that such parts, when they are hardened throughout, have a tendency to fail under sudden or fluctuating mechanical stress.

It is therefore well established in the prior are that steel parts which are subjected to the above-mentioned operating conditions should have a hard outer surface which gives them resistance against wear and abrasion, and a tough, resilient core which gives them the necessary mechanical resistance. In the past it has been customary to select the type of steel alloy for such machine elements on the basis of the intended hardening procedure which in turn would be determined by the heat and wear condition on the one hand, and by the operational stress conditions on the other hand. In the specific application of plastification cylinder for injection molding machines, for example, it has heretofore been customary to harden the inner wall of the plastification cylinder through a nitration treatment. The steel alloys which are suitable for this treatment are primarily chromium-alloyed, tempered steels which also contain aluminum for an increased formation of special nitrides. On the other hand, it is also possible to use other kinds of alloyed steels, high-speed steels or high-alloy tool steels of known composition.

The earlier-mentioned nitration treatment can be obtained in a gas atmosphere, but this procedure has the disadvantage that only a very small hardness penetration is obtained. This process is also very timeconsuming. For example, gas nitration at a temperature of 500C over a period of 90 hours produces a hardness depth of only 0.7 mm, whereas a similar nitration treatment over a period of 25 hours produces a hardness depth of only approximately 0.25 mm. Apart from the fact that these large time requirements for gas nitration are uneconomical, it was found that the relatively small hardness depths of these machine elements provide only a comparatively short useful life under conditions of severe wear and abrasion. In the case of nitration hardening, it was also found that the border zones of the machine elements may exhibit inadequate resiliency against impact forces, with the result that these brittle border zones chip under stress, which condition in turn leads to accelarated errosion in the chipped zone.

Comparable shortcomings were found in the case of bath-nitrated parts which were found to have similar inadequacies, especially in the case where the part is subjected to a high contact pressure against another part. Although in this case the treatment time required is considerably shorter, the resistance values obtained through bath nitration of the machine elements are still not satisfactory for such applications as plastification cylinders or feed screws of injection molding machines in which plastic materials with a mineral filler are processed. Based upon past experience, it can also be said that even the use of high-speed steel or high-alloy tool steels as a material for the above-mentioned machine elements does not provide sufficient longevity of these elements under the existing wear and stress conditions. Various additional experiments have also shown that, even when the hardening depth is increased substantially by extending the time of nitration treatment, no basic improvement was noticeable.

In view of the evident shortcomings of the earliermentioned nitration treatment of machine elements, it has also become common practice to resort to quenching of the parts. In this case, the necessary heating of the part to the transformation point Ac is obtained through induction heating or through flame heating. Hardening depths of more than 1 mm are normally obtained with induction heating in the medium-frequency range (0.5 to 20 kHz), while hardening depths of less than 0.5 mm are normally obtained through induction heating in the high-frequency range (40 to 6,000 kHz). It is known that this kind of heat treatment has already been used in practical applications for the production of feed screws for injection molding machines which, because of their non-cylindrical shape, have been treated by medium-frequency induction heating. However, severe wear conditions were again encountered when this type of feed screw was used for extended periods of time in the processing of plastic masses with a mineral filler, or which contain glass fibers.

The inadequate longevity of these machine elements can be primarily explained by the fact that, according to the present state of the art, only those steels are suitable for flame hardening or induction hardening which are, without exception, of the low-alloy rolling steel or forging steel type. These steels cannot attain the necessary wear resistance, as required in the abovementioned application. In the case of inductionhardened steels, they are presently limited to a carbon content of less than 0.6 percent, and their content of alloy components which form carbides or super carbides is less than 2.5 percent (see, for example, TABLE N 15-2 in Verein Deutscher Eisenhuttenindustrie, ed.: Werkstoff-Handbuch Stahl und Eisen, 4th edition, 1965).

Medium-alloyed and high-alloyed steels are normally unsuitable for induction hardening. There exist several progressively worsening drawbacks in the application of induction heat for the hardening of higher-alloyed steels: On the one hand, the heat conductivity decreases in reverse relation to the increasing content of alloy components, especially in the lower and medium temperature range, so that the rate of temperature increase must be greatly reduced, in order to avoid heating cracks. On the other hand, longer periods of temperature maintenance are required, in order to obtain a sufficient austenitization, because larger amounts of carbides have to be dissolved. Even an increase in the austenitization temperature cannot decrease these treatment times sufficiently to create acceptable heating conditions for induction heating. Lastly, there exists the risk of crack formation in this type of high-alloy parts during the quenching operation.

Some research on the structure and on the mechanical characteristics of high-frequency-hardened ball bearing steels has been conducted as early as 1957. However, these experiments have not produced any re sults suitable for practical application in this art, and more particularly, they have failed to change the heretofore established body of opinion, according to which it was impossible to treat high-alloy steels by induction hardening. Inasmuch as induction hardening was in fact applied to machine elements in such applications as feed screws for injection molding machines, the steels used for such parts were in all cases made of low-alloy rolling steel or forging steel.

It was also found that even the specialized wearresistant steels, which are characterized by a special chemical composition giving them a molecular structure suitable for high wear resistance, are generally inadequate in cases where the machine elements in question are subjected to the continuous abrasive influence of sinter masses or of plastic masses having a high mineral content, for example. The above applies particu larly to the austenitic steels. Thus, it has already been proposed to solve this problem in the case of plastification cylinders for injection molding machines by resorting to a cylinder design in which a through-hardened sleeve of high-alloy tool steel is incorporated in the cylinder. Obviously, such a design entails various production-related complications and cost problems. Furthermore, this type of design is subject to certain limitations as regards the maximum length in which such a sleeve can be produced, the narrow tolerances of the sleeve and of the plastification cylinder requiring a considerable amount of additional effort and cost.

SUMMARY OF THE INVENTION It is a primary objective of the present invention to provide machine elements of the earlier-mentioned type which do not have the above shortcomings and which combine a tough, resilient core for a high resistance against contact pressure and impact forces with a hard surface for an extremely high resistance against wear and heat, notwithstanding their comparatively low material and production costs.

The present invention proposes to attain the above objective by suggesting that the machine elements in question be fabricated of a high-alloy steel having a carbon content of at least 0.30 percent and a total content of alloy components for carbide and super carbide formation (Cr, W, V, and Mo) of more than 2 percent, the part having a hardening depth of at least 2 mm, containing carbides and super carbides in this surface zone which are created when this surface is first heated above the transformation point AC3 through induction heating or through flame heating, whereupon it is quenched.

The steel in question preferably has a total content of ailoy components of the carbide and super carbide forming type (Cr, W, V, and Mo) of more than 3.5 percent, the carbon content being more than 1.5 percent.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The novel method of heat treatment involved in the production of the machine elements of the invention is characterized by a heating operation in which the surface zone which is to be hardened is heated over a depth of at least 2 mm to a temperature above the transformation point Ac by means of induction heating or flame heating, whereupon the part is quenched.

The heating operation is preferably performed in successive steps, whereby the workpiece is progressively advanced into the heating zone, e.g. the induction zone, with time intervals of at least ten seconds between the advancing steps during which no heat is applied, the overall heating period during which the surface in question is maintained at a temperature above the transformation point being at least seconds.

The quenching operation should not be too abrupt, and should therefore not be performed in water. This operation may start while the workpiece advances in the last heating pass.

The heat treatment method of the invention effec tively destroys the previously accepted body of opinion in this art according to which inductive hardening or flame hardening is limited to the kinds of steels which are listed in the commonly available tables for induction hardening and flame hardening, and that in the case of alloy steels, this method is generally limited to machine elements of simple shape, as in the case of ball bearing steels, or in exceptional cases, in connection with steels having a high chromium content. Thus, according to these accepted beliefs, induction hardening and flame hardening were not industrially practical in connection with high-alloy, high-carbon steels, much less in the case of machine elements of such steels whose surface configuration considerably deviates from the cylinder shape, as is the case with feed screws for injection molding machines, for example.

The apparatus requirements for this novel heat treatment method can be simplified by arranging the successive passes during the heating operation in such a way that the inductive heating during each pass is performed at an unchanged energy level. However, in some cases it may be necessary to apply progressively higher energy levels during successive inductive heating passes following each heating pause, in order to obtain an optimal end product.

In the following are described several specific examples of machine elements suggested by the invention and produced in accordance with the novel method disclosed herein.

EXAMPLE 1 The feed screw for an injection molding machine having a maximum diameter of 22 mm was machined out of a cold work tool steel of the following composition: C 2.20%; Si 0.25%; Mn 0.30%; P and S 0.03%; Cr 12.00%; Mo 0.90%; and V 2.20% (designation under DIN X 220 standard: CrVMo 122).

Inductive hardening was performed with the aid of a medium-frequency transformer. The part was heated above the transformation temperature of approximately 1250C in a three-pass operation. During each pass the feed screw was advanced through the heating coil along its entire length. The energy level during all three passes remained unchanged. Pauses of between 40 to 80 seconds were made between passes. The temperature levels obtained within the heating zone of approximately 4 to 5 mm depth for each heating pass were approximately as follows: 500C following the first pass, 800C following the second pass, and 1 100C following the third pass. The quenching operation was initiated during the third pass, the duration of quenching lasting 1 minute. A hardening depth of 4 mm was obtained.

EXAMPLE 2 A plastification cylinder for an injection molding ma chine having a bore diameter of 22 mm was machined from a hot work tool steel of the following composition: C 1.65%: Si 0.30%; Mn 0.30%; P and S 0.03%; Cr 12.00%; Mo 0.60%; V 0.10%; and W 0.50% (designation under DIN X 165 standard: CrMoV 12).

The inductive hardening operation was performed by means of a high-frequency generator. The surface zones of the plastification cylinder which were to be hardened were heated above the transformation point of approximately 1100C in four successive heating steps. Each heating step involved a separate pass.

The surface which was to be hardened was heated over a depth of approximately 4 mm, the approximate temperature levels after each pass being: 400C after the first pass, 600C after the second pass, and 1 100C after the third pass. The overall duration of heating was approximately 2.8 minutes. Heating pauses of approxi mately 40 seconds were made between the several heating passes. The energy level applied during these passes was each time increased over the level of the preceding pass.

The quenching operation involved a water spray which was initiated during the last heating pass. The overall duration of quenching was approximately 1.5 minutes.

In a manner comparable to the above-described examples No. 1 and 2, special machine elements such as the mentioned plastification cylinder and feed screw may also be made of the following hot work too] steels:

a. C 0.40%; Si 1.00%; Mn 0.40%; P and S 0.025%;

Cr 5.30%; Mo 1.40%; and V 1.00%;

b. C 0.40%; Si 1.00%; Mn 0.40%; P and S 0.025%;

Cr 5.20%; V 0.15%; and W 3.50%;

c. C 0.60%; Si 0.30%; Mn 0.30%; P and S 0.025%; Cr 4.00%; Mo 0.90%; V 0.70% and W 9.00%.

Also found to be useful for this purpose were cold work tool steels of the following composition:

d. C 1.65%; Si 0.30%; Mn 0.30%; P and S 0.03%;

Cr 12.00%; Mo 0.50%; and Co 1.30%; e. C 2.10%; Si 0.25%; Mn 0.30%; P and S 0.03%;

Cr 12.00%; Mo 0.40%; W 0.70%; and Co 1.00%.

In some of the tests performed with these steels it was found that minor distortions in shape occured during treatment, especially in the case where heating to the transformation point was done too quickly. However, in each case it was possible to straighten out the workpiece. It was also found to be advantageous to anneal the finished workpieces to a temperature of 550C. The

machine elements obtained in this manner proved to have an extremely high resistance against erosion, and their corrosion resistance in the temperature region of to 600C was found to be good. The wear resistance was tested in comparative wear tests. A comparison was made between the injection unit of an injection molding machine which had a plastification cylinder and a feed screw made of previously known steels and similar injection unit where the cylinder and the feed screw were made of surface-hardened steel in accordance with the present invention. Both units were used to inject plastic masses containing glass fibers.

The erosion of the workpieces was measured after 300 hours of operation and after 600 hours of operation by determining the weight loss of the parts. In each case it was found that the weight loss of the plastification cylinder made of nitrated steel was a multiple of the weight loss of a comparable plastification cylinder made in accordance with this invention. Similar results were obtained by comparing feed screws made of alloy tool steel in accordance with the invention. The prior art feed screw had been machined of an alloy steel which was suitable for induction hardening in accordance with established reference tables (Table 4 in Die Praxis der induktiven Warmbehandlung, 1961, page 32)v From the test results obtained thus far it can be concluded that the useful life of plastification cylinders and feed screws for injection molding machines produced in accordance with the invention is between two and ten times the useful life obtainable from similar parts made in accordance with prior art technology.

It should be understood, of course, that the foregoing disclosure describes only preferred embodiments of the invention and that it is intended to cover all changes and modifications of these examples of the invention which fall within the scope of the appended claims.

1 claim:

1. For use as plastification feed screw or a plastification cylinder of a plastics processing machine, especially an injection molding machine, a machine element of steel having a hard, highly abrasion resistant surface and a tough, resilient core for high resistance against mechanical stress and wherein:

the steel from which the machine element is made is a high-alloy tool steel having a carbon content of no less than 1.5 percent and a total content of carbide and super carbide forming alloy components of no less than 3.5 percent, said alloy components being materials selected from the group consisting of chromium, tungsten, vanadium, and molybdenum; and

at least a portion of the surface of the machine element which is subjected to wear is surfacehardened to a depth of at least 2 mm, by multi-pass induction heating said surface to a temperature above the transformation point Ac: and by subsequently quenching the machine element.

2. A machine element as defined in claim 1, wherein:

said carbide and super carbide forming alloy components include a chromium content of more than 2.5 percent and a molybdenum content of more than 0.2 percent.

3. A machine element as defined in claim 1, wherein:

the steel from which the machine element is made contains alloy components in the following percentages:

carbon (C) 2.20% silicium (Si) 0.25% manganese (Mn) 0.30% phosphorus (P) and sulfur (S) 0.03% chromium (Cr) 12.00% molybdenum (Mo) 0.90% vanadium (V) 2.20%

4. A machine element as defined in claim 1, wherein: the steel from which the machine element is made contains alloy components in the following percentages:

carbon (C) 1.65% silicium (Si) 0.30% manganese (Mn) 0.30% phosphorus (P) and sulfur (S) 0.03% chromium (Cr) 12.00% molybdenum (Mo) 0.60% vanadium (V) 0.10% tungsten (W) 0.50%

S. A method of producing a plastification feed screw for the processing of abrasive plastic raw materials in an injection molding machine, or the like, comprising the steps of:

machining the feed screw from a piece of tool steel alloy containing no less than 1.5 percent carbon and no less than 3.5 percent carbide and super carbide forming alloy components;

heating the feed screw surface in an induction field,

by moving it in relation to a surrounding induction coil whose frequency is comprised between 0.5 and 20 kHz, in at least two successive passes, thereby progressively heating the feed screw to a temperature above the transformation point Ac over a depth of at least 2 mm; and thereafter quenching the heated feed screw, by passing it through a quenching means surrounding the feed screw.

6. A method as defined in claim 5, wherein:

the step of heating includes a pause of at least 10 seconds between successive heating passes; and

the successive heating passes are adjusted for the attainment of progressively higher temperatures. 7. A method of producing a plastification cylinder for the processing of abrasive plastic raw materials in an injection molding machine, or the like, comprising the steps of:

machining the plastification cylinder from a piece of tool steel alloy containing no less than 1.5 percent carbon and no less than 3 percent carbide and super carbide forming alloy components, the machined cylinder having a straight throughbore;

heating the cylinder bore surface in an induction field, by moving it in relation to an induction coil which fits into the cylinder bore and whose fre quency is comprised between 40 and 6000 kHz, in at least two successive passes, thereby progressively heating the plastification cylinder to a temperature above the transformation point Ac over a depth of at least 1 mm; and thereafter quenching the heated plastification cylinder, by moving it in relation to a quenching means.

8. A method as defined in claim 7, wherein:

the step of heating includes a pause of at least 10 seconds between successive heating passes; and

the successive heating passes involve the application of progressively higher induction energy levels. 

1. For use as plastification feed screw or a plastification cylinder of a plastics processing machine, especially an injection molding machine, a machine element of steel having a hard, highly abrasion resistant surface and a tough, resilient core for high resistance against mechanical stress and wherein: the steel from which the machine element is made is a high-alloy tool steel having a carbon content of no less than 1.5 percent and a total content of carbide and super carbide forming alloy components of no less than 3.5 percent, said alloy components being materials selected from the group consisting of chromium, tungsten, vanadium, and molybdenum; and at least a portion of the surface of the machine element which is subjected to wear is surface-hardened to a depth of at least 2 mm, by multi-pass induction heating said surface to a temperature above the transformation point Ac3 and by subsequently quenching the machine element.
 2. A machine element as defined in claim 1, wherein: said carbide and super carbide forming alloy components include a chromium content of more than 2.5 percent and a molybdenum content of more than 0.2 percent.
 3. A machine element as defined in claim 1, wherein: the steel from which the machine element is made contains alloy components in the following percentages:
 4. A machine element as defined in claim 1, wherein: the steel from which the machine element is made contains alloy components in the following percentages:
 5. A METHOD OF PRODUCING A PLASTIFICATION FEED SCREW FOR THE PROCESSING OF ABRASIVE PLASTIC RAW MATERIALS IN AN INJECTION MOLDING MACHINE, OR THE LIKE, COMPRISING THE STEPS OF: MACHINING THE FEED SCREW FROM A PIECE OF TOOLSTEEL ALLOY CONTAINING NO LESS THAN 1.5 PERCENT CARBON AND NO LESS THAN 3.5 PERCENT CARBIDE AND SUPER CARBIDE FORMING ALLOY COMPONENTS; HEATING THE FEED SCREW SURFACE IN AN INDUCTION FIELD, BY MOVING IT IN RELATION TO A SURROUNDING INDUCTION FIELD, BY WHOSE FREQUENCY IS COMPRISED BETWEEN 0.5 AND 20 KHZ, IN AT LEAST TWO SUCCESSIVE PASSES, THEREBY PROGRESSIVELY HEATING THE FEED SCREW TO A TEMPERATURE ABOVE THE TRANSFORMATION POINT AC3, OVER ADEPTH OF AT LEAST 2 MM; AND THEREAFTER QUENCHING THE HEATED FEED SCREW, BY PASSING IT THROUGH A QUENCHING MEANS SURROUNDING THE FEED SCREW.
 6. A method as defined in claim 5, wherein: the step of heating includes a pause of at least 10 seconds between successive heating passes; and the successive heating passes are adjusted for the attainment of progressively higher temperatures.
 7. A method of producing a plastification cylinder for the processing of abrasive plastic raw materials in an injection molding machine, or the like, comprising the steps of: machining the plastification cylinder from a piece of tool steel alloy containing no less than 1.5 percent carbon and no less than 3 percent carbide and super carbide forming alloy components, the machined cylinder having a straight throughbore; heating the cylinder bore surface in an induction field, by moving it in relation to an induction coil which fits into the cylinder bore and whose frequency is comprised between 40 and 6000 kHz, in at least two successive passes, thereby progressively heating the plastification cylinder to a temperature above the transformation point Ac3; over a depth of at least 1 mm; and thereafter quenching the heated plastification cylinder, by moving it in relation to a quenching means.
 8. A method as defined in claim 7, wherein: the step of heating includes a pause of at least 10 seconds between successive heating passes; and the successive heating passes involve the application of progressively higher induction energy levels. 